Air-to-air heat pump systems are heat moving devices used in residential and commercial applications. Heat is absorbed in an evaporator in a first location and released in a condenser in a second location. The systems are designed so that operations can be reversed so that an area can be either cooled or heated. Thus, on reversal of the heat flow direction, the evaporator at the first location becomes a condenser; and the condenser at the second location becomes an evaporator.
During the heating cycle, the outdoor unit acts as an evaporator and the indoor unit acts as a condenser. Moisture from the outdoor air will condense on the outdoor coil. As the ambient temperature decreases below about 45° F., the outdoor coil temperature will rapidly approach 32° F. or lower, causing the condensed moisture to turn to ice. The ice restricts the airflow across the coil, which in turn affects the ability of the evaporator to efficiently perform its function of absorbing heat from the ambient air as the refrigerant fluid undergoes a phase change when at least a portion of the refrigerant fluid is converted from a liquid state into a gaseous state. The formation of the ice thus reduces the performance or efficiency of the heat pump system. In order to restore performance, the system will enter an evaporator defrosting cycle. The defrosting cycle on some heat pumps begins with a timed period of supplemental electric heat applied to the frosted or iced coil by de-icing electric heating elements. Also in common use today are defrost controls. These are based upon temperature differentials, pressure differentials or a combined time/temperature differential. These units reverse the operation of the heat pump so that the flow of hot refrigerant is reversed, flowing in the opposite direction than required for heating, that is, flowing directly from the compressor to the outdoor unit in order to heat the outdoor unit. There are many variations of how this is accomplished. One such device is described in Trask, U.S. Pat. No. 4,843,838 issued Jul. 4, 1989. However, while the unit is in such a defrost cycle, it is not providing heat as the refrigerant flow is in the direction for cooling. If there is still a heat demand required in the space being heated, the heat demand typically is satisfied with supplemental electric resistance heat, which is expensive in comparison to the cost of running a heat pump.
Different bypass methods and apparatus for defrosting or de-icing have been taught. McCarty, U.S. Pat. No. 4,158,950 issued Jun. 26, 1979, discloses a bypass arrangement in which defrosting is accomplished by refrigerant after the compressor has stopped operation and any pressure differential within the system is equalized. Thus, operation of the heat pump system cannot be accomplished during the de-ice cycle and auxiliary heat solely must be relied upon to heat any designated areas during the de-icing operation.
In Chrostowski et al., U.S. Pat. No. 4,389,851 issued Jun. 28, 1983, a combination of reverse and nonreverse defrost is utilized to de-ice the heat exchanger. During de-icing, a three way valve directs gas from the compressor to an outdoor coil. The only heat exchange path during the defrost mode is from the compressor to the outdoor unit. A valve closes to prevent the flow of refrigerant between the indoor unit and the outdoor unit. This valve and a reversing valve isolate the indoor unit from the outdoor unit as refrigerant from the compressor defrosts the outdoor coil.
Bonne, U.S. Pat. No. 4,441,335, issued Apr. 10, 1984, is similar to Chrostowski et al. in that the bypass arrangement moves discharge refrigerant from the compressor directly to the outdoor coil. In addition to utilizing a plurality of three way valves to direct the flow of the refrigerant, Bonne provides no circuit between the indoor unit and the outdoor unit in which the refrigerant is not first required to pass through an expansion valve, thereby lowering its pressure.
Sato et al., U.S. Pat. No. 4,519,214 issued May 28, 1985, utilizes a branch circuit for the defrost cycle that passes hot compressor refrigerant through the outdoor unit to de-ice the outdoor coil. However, to accomplish this task, the cycle is first reversed, thereby causing the air-to air heat pump to be placed into the cooling mode and converting the outdoor unit into a condenser. The refrigerant fluid passes through the outdoor coil/condenser and back to the compressor until defrost is accomplished.
Aoki et al, U.S. Pat. No. 4,760,709 issued Aug. 2, 1988, utilizes a five-way valve to direct a portion of hot refrigerant gas from the compressor to the outdoor unit to accomplish defrost of the outdoor unit, while continuing a flow of the remaining refrigerant from the compressor to the indoor unit so that the heat pump can continue to provide heat during the defrost cycle. After the refrigerant leaves the indoor unit, it passes to the outdoor unit/evaporator through an expansion valve in the usual manner. There is no other connection or branch between the indoor and outdoor unit.
An arrangement of utilizing refrigerant leaving the indoor unit and indoor coil for a defrost/de-ice cycle would be effective in making use of relatively high pressure refrigerant having a temperature significantly higher than that of the outdoor ambient temperature or the outdoor coil. Such an arrangement would not seriously impact the heating functions of the air-to-air heat pump and would eliminate the need to reverse the operation of the heat pump. It would also eliminate or reduce the need to rely on supplemental auxiliary heat during the defrost cycle. A simple arrangement that utilizes minimal and readily available equipment is desirable to keep manufacturing costs low. Furthermore, a unit having predetermined set points that can be changed simply by a user is also desirable to increase the flexibility of the system as a result of the environment in which it is installed.